As discussed in the above-incorporated U.S. patent applications, x-ray analysis methods provide some of the most significant developments in twentieth and twenty-first century science and technology. The use of x-ray fluorescence, x-ray diffraction, x-ray spectroscopy, x-ray imaging, and other x-ray analysis techniques has led to a profound increase in knowledge in virtually all scientific fields.
X-ray fluorescence (XRF) is an analytical technique by which a substance is exposed to a beam of x-rays to determine, for example, the presence of certain components. In XRF, at least some of the chemical constituents of the substance exposed to x-rays can absorb x-ray photons and produce characteristic secondary fluorescence. These secondary x-rays are characteristic of the chemical constituents in the substance. Upon appropriate detection and analysis these secondary x-rays can be used to characterize one or more of the chemical constituents. XRF techniques have broad applications in many chemical and material science fields, including medical analysis, semiconductor chip evaluation, and forensics, among others.
One emerging application for such measurement techniques is the detection of sulfur in fuels. Sulfur in transportation fuels is emitted as SO2 or SO3, which typically forms sulfuric acid in the atmosphere, and some of which forms ammonium sulfate or ammonium bisulfate. These sulfur compounds are the dominant contributor to PM 2.5 pollution. Although there are other human-based sources of sulfur, transportation fuels have been a major contributor. In New York City more than half the sulfur in air pollution is attributable to transportation sources. Sulfur in fuels poisons catalytic converters, and reductions in sulfur levels in fuels also reduce other pollutants from transportation sources. To address these problems, the U.S. Environmental Protection Agency (EPA) has recently mandated a reduction of sulfur in on-road diesel fuel from the current level of 500 ppm to 15 ppm by 2006. The EPA estimates that the rule will annually prevent over 8,000 premature deaths and tens of thousands of cases of bronchitis and asthma in the U.S. Europe and Japan are making similar changes in approximately the same time frame.
The petroleum industry has demonstrated the ability to remove sulfur from highway fuels. However, controlling fuel production and distribution are problematic, as there are no robust methods for on-line measurement of sulfur level in fuels during their processing and distribution. To meet the 15 ppm mandated levels, approximately 7-8 ppm must be measured at refineries to account for contamination during transportation. To attain good statistical control and to monitor feedstocks with lower than average sulfur levels, the limit of detection may need to be less than 1 ppm.
XRF techniques can be used for this application (as discussed above and throughout the above-incorporated applications). The basic technique involves exciting a fuel sample with x-rays and examining the fluorescence emitted. Each element emits a unique spectral signature. A detector then measures the wavelengths of the emitted x-rays, and software can reduce this measured spectrum to a weighted composition of the sulfur in the sample.
XRF fluid testing can take place off-line, i.e., using a bench-top, laboratory-type instrument to analyze a sample. The material is removed from its source (e.g., for fuel, from a refinery or transportation pipeline) and then simply deposited in a sample chamber. Off-line instruments need not meet any unusual operational/pressure/environmental/size/weight/space/safety constraints, but merely need to provide the requisite measurement precision for a manually-placed sample. Moreover, off-line instruments can be easily maintained between measurements.
On-line analysis offers the potential of “real-time” monitoring of sample composition at various points in the manufacturing process. For example, all fuel product is subject to the EPA rules discussed above—requiring some variant of on-line monitoring during fuel refining and transportation in pipelines. On-line analysis of fuels in a refinery and in pipelines, however, requires consideration of numerous operational issues not generally present in an off-line, laboratory setting. A fully automated fuel sample handling system is required—with little or no manual intervention or maintenance. Also, since fluids are usually under pressure in pipelines, any sample handling system must account for pressure differentials. This is especially important since certain portions of XRF x-ray “engines” (discussed further below) may operate in a vacuum. Also, the instrument's electronics require packaging in an explosion-proof housing—separate from the sample handling system.
In this application, therefore, one of the most critical components is the sample barrier which allows photons of x-rays to excite sulfur atoms in the fluid, and photons emitted from the atoms to be counted at the engine's detector, while at the same time maintaining the vacuum or atmosphere in the x-ray engine and the pressure of the fluid. The present inventors have discovered that x-ray stimulation creates sulfur ionization and adsorption at this interface over time and on certain types of barrier materials—leading to undesired sulfur residue and degradation of the barrier's x-ray transparency. More generally, many XRF applications require a barrier to protect the engine from any number of adverse interface effects from the sample material and/or the measurement environment.
Therefore, any barrier technique in an on-line system should meet certain criteria: transparency—i.e., the transmission of x-rays with the minimum amount of x-ray absorption; strength—the barrier material must be strong enough to support, e.g., fluid sample pressures of 20-100 psi or more from continuous flows in a pipeline; and finally, contamination—the technique must address contamination of the barrier from the sample material and/or the measurement environment.
What is required, therefore, is a barrier technique and apparatus for an on-line x-ray analysis system, which protects the x-ray engine from adverse sample and environmental effects, while maintaining the integrity and transparency of the interface to the sample for accurate measurements.